Hitherto, as a construction machine or the like, an axial hydraulic piston pump which is driven by an engine or an axial hydraulic piston motor which is driven by a high-pressure hydraulic fluid has been widely used.
For example, the axial hydraulic piston pump includes a cylinder block which is provided so as to rotate along with a rotation shaft rotatably provided inside a casing and has a plurality of cylinders extending in the axial direction while being away from each other in the circumferential direction, a plurality of pistons which are slidably inserted and fitted into the respective cylinders of the cylinder block and move in the axial direction with the rotation of the cylinder block so as to suction or discharge a hydraulic fluid, and a valve plate which is provided between the casing and an end surface of the cylinder block and is provided with a suction port and a discharge port communicating with the respective cylinders. Then, when the driving shaft of the hydraulic pump is rotationally driven, the cylinder block rotates along with the operation shaft inside the casing and the pistons move in a reciprocating manner to the respective cylinders of the cylinder block so as to pressurize the hydraulic fluid suctioned from the suction port into the cylinder by the pistons so that the hydraulic fluid is discharged as a high-pressure hydraulic fluid to the discharge port.
Here, when the cylinder port of each cylinder communicates with the suction port of the valve plate, the piston moves in a direction in which the piston protrudes from the cylinder from the start end to the terminal end of the suction port, thereby performing a suction process of suctioning the hydraulic fluid from the suction port into the cylinder. Meanwhile, when the cylinder port of each cylinder communicates with the discharge port, the piston moves in a direction in which the piston advances into the cylinder from the start end to the terminal end of the discharge port, thereby performing a discharge process of discharging the hydraulic fluid inside the cylinder into the discharge port. Then, when the cylinder block rotates so that the suction process and the discharge process are repeated, the hydraulic fluid suctioned from the suction port into the cylinder by the suction process is pressurized by the discharge process so that the hydraulic fluid is discharged to the discharge port.